Ratio R_P∕p between the polarized emission at 353 GHz, P₃₅₃, and the polarization degree, p_V, measured in extinction in the V band, as a function of the grain axis ratio ϵ, for astrosilicates. Labels indicate the grain porosity. The mean value of R_P∕p in translucent lines of sight (Paper I), together with its range of uncertainty, is indicated. Blue triangles correspond to an astrosilicate matrix with a-C inclusions (6% in volume). Blue squares correspond to the Draine & Fraisse (2009) dust models (oblate grains with ϵ = 1.4 and ϵ =1.6).